Coated gypsum board products and method of manufacture

ABSTRACT

A method of producing a coated gypsum board includes depositing a gypsum slurry to form a wet gypsum board having a gypsum core; applying a coating to the wet gypsum board; and drying the wet gypsum board, wherein the step of applying the coating occurs prior to the step of drying and the coating penetrates through at least a portion of the facing sheet and into at least a portion of the gypsum core. A board produced by the method is made with a coating that matches a joint compound.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of PCT InternationalApplication No. PCT/US02/02106, filed on Jan. 25, 2002, and whichdesignated the United States; the present application also claims thebenefit of U.S. Provisional Application No. 60/264,068, filed on Jan.26, 2001. The present application is also a continuation-in-part of U.S.patent application Ser. No. 10/308,084, filed Dec. 3, 2002, which is acontinuation-in-part of U.S. patent application Ser. No. 09/633,264,filed on Aug. 4, 2000, now abandoned. The disclosure of all of theabove-referenced applications are hereby incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to coatings for gypsum boardproducts, and the production thereof. More particularly, the presentinvention relates to the composition of coatings for gypsum boardproducts, and methods of producing the coatings and applying thecoatings in one continuous gypsum board manufacturing process.

2. Discussion of Related Arts

In the discussion of the related art that follows, reference is made tocertain structures and/or methods. However, the following referencesshould not be construed as an admission that these structures and/ormethods constitute prior art. Applicants expressly reserve the right todemonstrate that such structures and/or methods do not qualify as priorart against the present invention, if appropriate.

The product density and surface type and finish of traditional gypsumboard products are manufactured to have a desired strength andappearance while minimizing product weight and costs. Generally,calcined gypsum slurry is agitated in a mixer with aqueous foamingagents and deposited onto a conveyor carrying a facing sheet. A backingsheet is applied over the slurry and the wet gypsum board product isformed to the desired thickness between forming rollers. The wet gypsumsets during the conveying process, is cut to the desired length, and isdried by passing the gypsum board product through a drying oven.

Coatings for gypsum board products may be used to impart surfacehomogeneity and improved surface appearance, both in the manufacturedcondition and after a decorative coating has been applied. An example ofa previously known two-step manufacturing method for coated gypsum boardproducts is disclosed in Australian Patent Application No. 19322/92, thedisclosure of which is incorporated herein by reference. A surfacecoating is applied to the facing sheet and dried before the gypsum isapplied to the facing sheet in the gypsum board manufacturing process.

An alternative method for applying a coating utilizes a separate coatingprocess subsequent to the manufacture and drying of the gypsum boardproduct. U.S. Pat. No. 6,105,325, issued to Zuber et al., the disclosureof which is incorporated herein by reference, discloses prefabricatedelements, preferably flat prefabricated elements such as gypsumfiberboards, cement fiberboards, cement wallboards or plasterboards,comprising a sheet of lining paper and a plaster body, wherein thestructure and/or the external surface of the prefabricated elements issubstantially homogenous with the composition of a jointing material,e.g., a sealing coat, a joint coat and/or a joint-pointing coat, in itsdry state, to obtain an overall surface having one or more physicalcharacteristics, including color or shade, which are substantiallyhomogeneous in virtually the entire overall surface, including in theregion of the visible outer face of the joints.

According to U.S. Pat. No. 6,105,325, in addition to the color or shade,at least any one of the following physical characteristics issubstantially homogenized between the flat prefabricated elements andthe jointing material, namely: the surface appearance, includingreflectance; the absorption of surface water; and decoloration orcoloration under the effect of natural light.

In the above processes, the coating imparts desirable aestheticqualities including surface coloration, reflectance, and absorbency.However, the methods are capital intensive, requiring separate anddedicated production facilities in which to apply the coating andsuffering inefficiencies by not being part of a larger in-lineproduction facility.

OBJECTS AND SUMMARY

It is an object of the present invention to provide an efficient systemfor manufacturing a gypsum board having a high quality finish.

The present invention provides a coated gypsum board which can beproduced in one continuous gypsum board manufacturing process. Suchgypsum board can be made either with paper on both sides thereof, paperon one side thereof, or without paper on either side thereof. Further,such gypsum board coatings may be applied to wet gypsum board prior todrying.

In an exemplary embodiment, a coated gypsum board comprises a gypsumcore having a first side and second side and a facing sheet disposed onthe first side. A coating is disposed on at least a portion of thefacing sheet and at least a portion of the coating penetrates into thefacing sheet and/or the gypsum core. In an additional embodiment, thegypsum board further comprises a backing sheet on the second side of thegypsum core. In one aspect, the coating may penetrate into the gypsumcore to a substantially uniform depth across an area of the gypsumboard. A gypsum board with such a coating exhibits a nail pull value ofgreater than 80 pounds.

In a method of producing a coated gypsum board, a gypsum slurry isdeposited to form a wet gypsum board, a coating is applied to the wetgypsum board, and the wet gypsum board is dried. The coating can beapplied directly to the gypsum core or to a facing sheet applied overthe gypsum slurry. When the coating is applied to the gypsum board priorto drying, the coating can penetrate into the facing sheet and/or thegypsum core and forms a coating that is up to 30 mils in thickness.

In one exemplary embodiment, the coating has a composition of 25-75 vol.% water, 30-70 wt. % calcium carbonate, 0-30 wt. % fillers, 2-10 wt. %latex emulsion, and 0-10 wt. % other additives. Fillers can comprise oneor more of mica, talc, clay and limestone. 2-8 wt. % perlite can beadded to the coating to form a lightweight product and pigment can alsobe added in an amount effective to provide a desired tint to thecoating.

In a further embodiment, a coating to be applied to a gypsum board has acomposition of 10-60 vol. % water, 50-90 vol. % calcined gypsum; 0.1-10vol. % binder; 0-50 vol. % limestone; 0-10 vol. % clay; 0-30 vol. %other fillers; and 0-10 vol % additives. Fillers can comprise one ormore of mica and talc. 2-15 vol. % perlite can be added to the coatingto form a lightweight product and pigment can also be added in an amounteffective to provide a desired tint to the coating.

In one exemplary embodiment, the coating to be applied to a gypsum boardis a joint compound or a diluted joint compound.

An exemplary method of making a wall includes depositing a gypsum slurryto form a wet gypsum board having a gypsum core, applying a coating tothe wet gypsum board, and drying the wet gypsum board. The step ofapplying the coating occurs prior to the step of drying the wet gypsumboard. The coated gypsum board is then fastened to a support structureto form the wall and the seams between adjacent coated gypsum boards aretaped and finished using a joint compound having a compositionsubstantially similar to the composition of the coating.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

Aspects and advantages of the invention will become apparent from thefollowing detailed description of preferred embodiments thereof inconnection with the accompanying drawings in which like numeralsdesignate like elements and in which:

FIG. 1 is a cross-sectional view of an embodiment of the inventionincluding a coated gypsum board with a coating applied to a facingsheet.

FIG. 2 is a cross-sectional view of an embodiment of the inventionincluding a coated gypsum board with a coating applied to the gypsumcore.

FIG. 3 is a schematic of an embodiment of a production lineincorporating a coating step prior to drying a gypsum board.

FIG. 4 is a schematic plan view of the coating step showing a coatingmachine.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Although the description herein may from time to time refer to awallboard, a ceiling tile, a ceiling board, or a gypsum board, in eachcase, the concepts of the present invention may be applied generally towallboards, ceiling boards, or ceiling tiles.

As used herein, the following physical characteristics are defined asfollows:

the reflectance factor of the overall surface, including that of thevisible outer face of the joints, is between 70% and 80%, and preferablybetween 72% and 76%, for a wavelength of 457 nm;the decoloration or coloration of the overall surface, including that ofthe visible outer face of the joints, has a color deviation (delta E*)at most equal to 3 after exposure for 72 hours to a source of UVradiation arranged at 15 cm from the surface and having a wavelength atleast equal to 290 nm; andthe surface water absorption of the overall surface, including that ofthe visible outer face of the joints, is not less than 60 minutes.

Generally, gypsum board is installed during construction on surfacessuch as wall, ceilings, and the like, in edge abutting engagement. Afteraffixing the gypsum board to the surface with suitable attachments(e.g., nails, screws, epoxy, and so forth), the seams and/or artifactsfrom the attachments (e.g., depressions from nails, nail heads, screwholes, and so forth) are covered with a coating to present a uniform andsmooth finish. For seams, said coatings can be used in combination withtape. The coating material is sometimes referred to as joint compound.

Coatings for use in the gypsum board industry can be of one of twotypes. In a first type, the coating is applied and the evaporation ofwater over time produces a hard coating. In a second type, the coatingis applied and allowed to set for a suitable time during which thecomponents of the coating solidify into a hard gypsum-based coating.

A coating of either of the first type or the second type can be appliedto one side of a gypsum board (e.g., applied to a facing sheet ordirectly to a gypsum core) during the manufacturing process to form acoated gypsum board. The selection of the coating can be made such thatthe coating of the coated gypsum board substantially matches the coatingor joint compound intended to be used to cover the seams and/orartifacts of attachment associated with the affixing of the coatedgypsum board to a structural support. In one embodiment, the coating isa diluted form of the coating or joint compound.

Conventionally, one side of the gypsum or plaster board is covered witha sheet of paper which has a dark color which can vary between a greycolor and a chestnut color, since it is composed of cellulose fiberswhich have not undergone any particular purifying treatment.Traditionally, this so-called grey paper is obtained from unbleachedchemical pulp and/or from mechanical pulp, and/or from thermomechanicalpulp and/or from semi-chemical pulp. By mechanical pulp, it is usuallymeant a pulp obtained entirely by mechanical means from various rawmaterials, essentially wood, which can be provided by salvaged productsoriginating from wood, such as old cardboard boxes, trimmings of kraftpaper and/or old newspapers. Thermomechanical pulp means a pulp obtainedby thermal treatment followed by a mechanical treatment of the rawmaterial. By semi-chemical pulp is meant a pulp obtained by eliminatingsome of the non-cellulose components from the raw material by means ofchemical treatment and requiring a subsequent mechanical treatment inorder to disperse the fibers. As used hereinafter, this type of paper isreferred to as grey paper.

The other side of the board is covered with a sheet of paper of a colorgenerally lighter than the grey sheet. To obtain this lighter color, thelayer or layers of this face are based on chemical pulp, ifappropriately bleached, composed of recycled and/or new cellulosefibers, and/or on mechanical pulp, if appropriately bleached. Bychemical pulp is meant a pulp obtained by eliminating a very largeproportion of the non-cellulose components from the raw material bychemical treatment, for example, by cooking in the presence of suitablechemical agents, such as soda or bisulphites. When this chemicaltreatment is completed by bleaching, a large part of the coloredsubstances is eliminated, as well as the substances which riskdecomposing by ageing and giving unpleasant yellow shades associatedwith the presence of, for example, lignin.

Plasterboards similar to Example 5 of document EP-A-0 521 804 areassembled by means of a conventional sealing joint, for example a jointcoat sold under the registered trade mark of “PREGYLYS”® of PLATRESLAFARGE. The upper web of the lining of the board is obtained from 65%bleached synthetic cellulose fibers and 35% talcum, and is covered witha pigment layer comprising, as mineral filler, 85% by weight of CaSO₄,2H₂O in the form of needles of a length of between 3 and 5 μm and, as abinder, 10.3% by weight of styrene-butadiene copolymer.

The sealing joint subsequently receives a thin layer of a joint-pointingcoat according to the invention, having the following composition: 50 to85% by weight of calcium carbonate, grain size from 5 to 35 μm, as amineral filler; 2 to 12% by weight of a binder comprising polyvinylacetates and acrylic acid esters in aqueous dispersion; 0.5 to 3% byweight of a silicone derivative as a hydrophobic agent; 0.1 to 0.9% of acellulose derivative of the methylhydroxyethylcellulose type; 0.1 to0.6% of a slipping agent of the attapulgite type; 1 to 12% of anothersilicate derivative as an additional slipping agent; 0.1 to 5% of apolycarboxylic acid ammonium salt as a dispersant; 0.001 to 0.015 of aniron oxide as a pigment; 0.1 to 0.3% of a preparation of N-fonroles andisothiazolinones as a biocide; 0.1 to 0.3% of a conventionalanti-foaming agent; water up to 100%.

The weight percentages given are in relation to the total weight of thecoat, unless indicated otherwise. For comparison requirements, standardboards conforming solely to French standard NF P 72-302 and notcomprising the above-defined upper web and pigment layer are assembledby means of a joint coat for a plasterboard of the range of coats“PREGYLYS”®, sold by the Company PLATRES LAFARGE. The characteristics ofthe two overall surfaces thus formed are compared by applying thefollowing tests:

(A) Degree of whiteness or reflectance factor R obtained according tostandard NFQ 03038 with a wavelength of 457 nm. This degree representsthe percentage ratio between of a reflected radiation of the body inquestion and that of a perfect diffuser under the same conditions.

(B) Surface water absorption: A drop of distilled water of a volume ofapproximately 0.05 cm³ at 23° C. is deposited on the surface. It isimportant that the drop be deposited and not allowed to fall from avariable height which consequently would crush it to a greater or lesserextent, thus falsifying the result. The duration in minutes representsthe surface absorption of the tested area.

(C) UV radiation resistance obtained by exposing the overall surfaces,in a cabinet comprising eight high pressure mercury vapour lamps, eachof 400 watts, to a wavelength which is not less than 290 nm. Thesurfaces are maintained at a distance of 15 cm from the lamps and at atemperature of 60° C. for 72 hours. The color deviations delta E* aremeasured on a spectrocolorimeter according to the standard DIN 6174 atan angle of 8.degree., illuminant D65 as a bright specular, included inthe system L*, a*, b*, in which L* is the luminance, a* represents thetransition from green to red, and b* represents the transition from blueto yellow. A point E* in this system, the said point being a function ofL*, a*, b*, defines the colorimetry of a sample and the deviation ismeasured in relation to a reference point. In general terms, a colordeviation beyond 2 becomes discernible to the naked eye.

The results of the tests (A) and (B) are collated in Table I and thoseof the test (C) are collated in Table II below.

TABLE I Overall surface according Standard overall surface to theinvention Reflectance R (%) Board: 50 to 60 Board: 72 to 76 Coat: 65 to85 Coat: 72 to 76 Absorption (min) Board: 50 Board: >=60 Coat: 15 Coat:>=60

This shows that the overall surface is clearly more homogeneous thanthat of an assembly according to the conventional technique. Moreover,the more homogeneous absorption time of the overall surface makes itpossible to use a paint having less covering capacity than thatnecessary with traditional boards and coats and is also beneficial tothe painting operation.

TABLE II Standard Invention Before Exposure Initial Measurements L* =82.94 L* = 90.41 of the board a* = −0.43 a* = −0.03 b* = 4.64 b* = 3.13Initial measurements L* = 90.70 L* = 89.70 of the joint a* = 0.73 a* =0.50 b* = 5.28 b* = 3.60 Board/Joint color deviation Board/Joint colordelta E* = 7.87 deviation delta E* = 1 Exposure to UV for 72 hoursMeasurements of the board L* = 81.10 L* = 90.38 after exposure a* = 0.69a* = −0.91 b* = 12.93 b* = 7.40 Color deviation delta E* = Colordeviation delta E* = 8.56; very substantial 4.36; substantial yellowingplus chestnut spots yellowing Measurements of the L* = 88.90 L* = 89.17joint after exposure a* = 0.91 a* = 0.50 b* = 3.83 b* = 3.19 Colordeviation delta E* = Color deviation delta E* = 2.32; slight yellowingplus a 0.67; very slight color few chestnut spots deviation

This table shows that the color deviation before exposure to UV is muchslighter for an overall surface according to the invention than for anoverall surface such as is obtained traditionally.

This table also shows that the change in the color deviation afterexposure to UV is much less pronounced in the overall surface accordingto the invention than traditionally. In fact, the color deviation beforeexposure and after exposure must be as little as possible, so that theoverall surface does not give the impression to the naked eye of beingspotted or being covered with zones of different shade and brightness.This is not possible with an overall surface obtained by means oftraditional plasterboards and products, but the very slight deviation ofthe overall surface according to the invention makes it possible tomitigate this disadvantage.

An embodiment of a coated gypsum board 100 is shown in FIG. 1. A gypsumcore 105 is disposed between a backing sheet 110 and a facing sheet 115.A coating 120 is disposed on the facing sheet 115. An alternativeembodiment of a coated gypsum board 200 is shown in FIG. 2, in which acoating 205 is directly disposed on a gypsum board 210. It should beclear that any combination of facing sheet and backing sheet may beutilized in practicing the invention. In cases where one or more sheetshave been removed, the coating may be applied directly to the gypsumcore with attendant penetration of the coating into the gypsum core.

The coating is evenly applied onto the surface of the gypsum board(e.g., applied to the facing sheet, if used, or directly to the gypsumcore) to a uniform thickness t that is preferably not sensitive tosurface irregularities. A typical thickness t for the coating is up to30 mils, preferably from 3-20 mils, and more preferably from 5-15 mils.The preferred thickness can be dependent on the ultimate application.For example, for wallboards, the thickness t is preferably 10 mils; forceiling tiles it is 20 mils. Furthermore, thicknesses outside of theseranges may also be contemplated by the present invention. However, acoating of approximately 30 mils or greater may result in cracks andgrazing of the surface finish when dried. Accordingly, thicker coatingsshould be used carefully.

In embodiments in which the gypsum board has a facing sheet, the coatingmay penetrate into at least a portion of the facing sheet and, in somecases, all the way through the facing sheet and into sections of thegypsum core, over at least a portion of the gypsum board. In oneembodiment, the coating may penetrate into the gypsum board oversubstantially the whole of the area to which it is applied, i.e, thewhole gypsum board, or predetermined portions if selected regions of thegypsum board have been coated.

The depth of penetration of the coating can be influenced by therelative moisture level and/or degree of set of the gypsum board,although the coating preferably penetrates into the gypsum board to asubstantially uniform depth. In the cross-sectional view as shown inFIG. 1, the coating 120 is shown to have penetrated into the facingsheet 115 and into the gypsum core 110 to a depth d. In other cases,i.e., where the moisture content is lower or the gypsum is in a state ofmore advanced set, the coating only penetrates a certain depth into thefacing sheet. In one embodiment of a gypsum board with a facing sheet,the depth of penetration into the gypsum core is from about 5 mils to 20mils, preferably from 10 mils to 15 mils. In embodiments of a gypsumboard without a facing sheet, the depth of penetration is from about 5mils to 30 mils, preferably from 10 mils to 20 mils.

A method of producing a coated gypsum board is shown in FIG. 3, in whicha flow chart of a coated gypsum board manufacturing line is depicted.The general manufacturing steps 300 include mixing 305 the gypsum slurryand forming 310 the gypsum into a desired shape, cutting 315 the gypsuminto the desired dimensions, followed by coating 320 to form a gypsumboard product and then drying 325. Setting of the gypsum occursprimarily between the forming step 310 and the cutting step 315. Themanufacturing steps from forming 310 to cutting, inclusive, are hereindescribed as the forming and setting line; the steps following cutting315 are herein described as the coating and drying line.

Starting at the forming and setting line, a formed, coated gypsum boardis manufactured. In a first embodiment, a gypsum core is coated on afirst surface with a coating while the gypsum core is wet. Inalternative embodiments, the gypsum board has a facing sheet and/or abacking sheet and is coated on a first surface, e.g., on the facingsheet surface, with a coating while the gypsum core is wet. For purposesof this application, a gypsum core is considered wet at any time duringthe manufacturing process before the gypsum board is passed through thedrying oven.

To control the setting time of the gypsum core, the components of thegypsum board can be adjusted, by, for example, controlling the quantityof accelerator and the retarder used in the composition of the gypsumslurry. The accelerator includes small crystal-like objects which arecoated with a starch or other dissolvable substances. As the coating onthe starch or other dissolvable substances is dissolved, the acceleratorcrystals form a starting point or seed for crystal growth of the gypsum.Retarder is preferably added to the gypsum board components in order todelay the set time of the gypsum core. The retarder has no long termeffect on the strength or other characteristics of the final boardproduct.

It is desirable to delay the set time under certain circumstances sothat the gypsum slurry does not set earlier than desired. For example,if the gypsum slurry begins to set in the mixer, inefficiencies result,such as blocked and/or clogged apparatus with attendant down time formaintenance and/or repairs. In addition, if the slurry sets beyond themixer but prior to forming, forming of the slurry may cause the crystalstructure to be disrupted during the setting process, which can resultin a weaker final board product. Accordingly, accelerator and retardermay be used singularly or in combination to adjust the set time of thegypsum to achieve a desired set or hardness at a desired time in themanufacturing process.

After forming, the gypsum board is conveyed by a conveying system alongthe forming and setting line to a cutting position. The conveying speedand distance are chosen in combination with the set time of the gypsumto result in gypsum having a desired set or hardness for subsequentcutting and manipulation. Cutting is accomplished by a cutting machine,as is conventionally known in the art. After being cut, the gypsum boardis turned over and placed on a conveying surface and transported to acoating and drying line. Where the gypsum core has a facing and/orbacking sheet, the manipulation of the gypsum board results in placingthe backing sheet in contact with conveying surface and exposing thefacing sheet.

On the coating and drying line, the gypsum board, with or without thefacing and/or backing sheet, is conveyed by the conveying means throughcoating machinery, which is located prior to the drying apparatus. FIG.4 schematically depicts an in-line coating process 400. The coatingmachinery 405 is positioned over the conveying path 410 of the gypsumboard 415. The conveying path typically includes a conveying belt 420 onwhich the gypsum board to be coated has been placed, although othersuitable conveying paths can be used, such as driven and nondrivenrollers. An uncoated and wet gypsum board 425 is passed under thecoating machinery 405 and a coating is applied via a coating applicator430. The coated gypsum board 435 is then conveyed to the nextmanufacturing steps, if any, and is then dried in, for example, an oven.

The coating can be applied using traditional coating machinery, such asa curtain coater. An example of a curtain coater is that provided byHymmen International, which can be custom designed to adapt to aparticular production line or manufacturing facility. The curtain coaterprovides a continuous curtain of the coating through which the wetgypsum board is passed. Alternative coating machinery may includesprayers, such as nozzle or shower sprayers, drip lines, and atomizationtechniques. An example of a spray technology is that provided by Graco,Inc. using spray tips suspended over the wallboard line. Anotheralternative method is a blade technology that scrapes the coating mix onto the surface of the gypsum board, i.e., on the surface of the gypsumcore or the facing sheet. In yet another embodiment, the coating can beapplied using one or more rollers.

Coatings may be applied to the gypsum board under any condition of setof the gypsum. For example, the coating can be applied before ovendrying, thus allowing the coating to penetrate into the wet facing sheetand/or the wet gypsum core during the conveying period. After coating,the coated gypsum board is dried and then prepared for storage andshipping. Alternatively, the coating can be applied after drying where,for example, the coating is heat sensitive or penetration into thefacing sheet is to be minimized.

Coatings can be applied to the whole surface area of the gypsum board,or to predetermined portions thereof. In the latter, predeterminedportions can be so designated by, for example, masking the gypsum boardto prevent coating of the designated sections or by adjusting thecoating machinery to only apply a coating to predetermined portions.

Several alternative embodiments of compositions of coating areenvisioned, each affecting the end product. One embodiment of a coating,referred to herein as the “rapid coating”, is a coating that hardens byevaporation of the water content, i.e., a drying coating. The rapidcoating comprises water; calcium carbonate; fillers such as limestone,mica, talc, and/or clay; binder; latex emulsion; and other additivessuch as preservative, and thickener. The rapid coating is provided witha water content of 35-45 wt. % and is diluted in the mixture to between10% and 60% dilution, e.g., to between 25-70 wt. %, preferably between39 wt. % and 56 wt. % water, by the addition of water. Table 3 presentsan exemplary formulation of an embodiment of the rapid coating.

TABLE 3 Component Amount (wt. %) water 25-75  calcium carbonate 30-70 mica 0-10 talc 0-10 clay 0-10 latex emulsion 2-10 other additives 0-10

In a lightweight embodiment of the rapid coating, the filler can includefrom 2% to 8% by weight perlite. An additional embodiment of the rapidcoating incorporates a pigment added in an amount effective to provide adesired tint to the coating. Additionally, the rapid coating may be madeso as to substantially match a property of the joint compound, such asthe absorbency, the reflectance, and so forth. The substantialhomogeneity of one or more properties may improve the aestheticappearance of the gypsum board in the finished condition after adecorative coating has been applied. In fact, according to oneembodiment, the coating applied to the board is actually a diluted formof the joint compound that is used to cover seams between the boards.

Another embodiment of the present invention uses a coating referred tohereinafter as the “setting coating”. The setting coating forms a hardgypsum-based coating by a setting process similar to that of the gypsumin the gypsum board. The setting coating comprises is a dry productcomprising calcined gypsum; binder; fillers such as limestone, mica,talc, and/or clay; and other additives such as preservative,accelerator, and thickener. The calcined gypsum may be either alpha orbeta type or a blend of these types. Prior to use the setting coating ismixed with 30-60 vol. % water to form a mixture. Table 4 presents anexemplary formulation of a setting coating.

TABLE 4 Component Amount (vol. %) Water 10-60  Calcined gypsum 50-90 Binder 0.1-10   Limestone 0-50 clay 0-10 Other fillers 0-30 Otheradditives 0-10

In a lightweight embodiment of the setting coating, the filler caninclude from 2% to 15% by volume perlite. An additional embodiment ofthe setting coating incorporates a pigment added in an amount effectiveto provide a desired tint to the coating.

The setting coating is generally more resilient to impact, i.e.,tougher, than the rapid coating, and may be utilized to provide an abuseresistant surface on the gypsum board, for example, for use in hightraffic areas or public spaces. Additionally, the coating may be made soas to substantially match a property of the joint compound, such as theabsorbency, the reflectance, and so forth. The substantial matching ofone or more properties may improve the aesthetic appearance of thegypsum board in the finished condition after a decorative coating hasbeen applied.

Binders suitable for use in the above rapid coating and setting coatinginclude starches, which assist the binding properties. Examples of suchbinders are wheat starch which can be preset between 0.01 and 5%.

A preservative suitable for use in the above rapid coating and settingcoating can be present between 0.01 and 5%. This additive prevents theproduct from spoiling prematurely, prior to application. The coatingscan spoil because there can exist inside the formulations water and/or anumber of organic raw materials. The preservative is also commonly knownas the biocide.

Accelerators may be used to adjust the set time in the above settingcoating. An example of an accelerator is potassium sulfate. Theaccelerator can be present in a suitable amount to achieve the desiredset time. Typically, the accelerator is present from 0.01 and 15% bysolids weight.

Thickeners, such as generic cellulose-based thickeners, may be used toadjust the rheological properties of the coatings and can be present insuitable amounts. Typical values are between 0.01 and 35% by solidsweight.

The penetration of the coating provided by the application of coatingsto a wet gypsum board results in improved coated gypsum boardproperties, such as better adhesion of the facing sheet to the gypsumcore. In one example, a coating improves board strength, as measured bynail pull. Table 5 includes nail hold values for both coated gypsumboard and uncoated gypsum board. From Table 5, it is shown that thecoating improves nail pull by up to 25% of nail pull values fortraditional uncoated gypsum board products.

TABLE 5 sample nail pull value coated 80 pounds uncoated 63 pounds

Additionally, a coated gypsum board displays a mechanical or strengthadvantage over uncoated gypsum board products that may allow forutilization of facing sheets of lower weight or quality with attendantcost reductions. Typically, facing sheets are anywhere between 40 to 60lbs. weight. In some applications, the facing sheet is a paper blendedwith mineral or synthetic fibers. If the facing sheet is to be coated,the improved board strength imparted by the coating allows the paperweight to be reduced. This advantage of the invention is realized whilestill obtaining the required handling characteristics to allowmanipulation without breakage during the manufacturing process.Furthermore, in view of the coating placed therein, the finished side ofthe board may be made with grey paper, instead of the more expensivebleached paper traditionally used for the finished side.

The Association of Wall and Ceiling Industries International (AWCI), theCeilings and Interior Systems Construction Association (CISCA), theGypsum Association (GA), and the Painting and Decorating Contractors ofAmerica (PDCA) have defined five levels of gypsum board finish.

The definitions provided for the various finishes are:

Level 1

A Level 1 finish is recommended in areas that would generally beconcealed from view or in areas that are not open to public traffic. InLevel 1, “set” has replaced “embedded” so that the first sentence nowreads, “All joints and interior angles shall have tape set in jointcompound.” This was changed to clarify that tape need not be coveredwith joint compound to fulfill the requirements of Level 1. In Level 1,the surface is left free of excess joint compound. Ridges and tool marksare acceptable for a Level 1 finish. This level is often specified inthe plenum area above ceilings, in attics, or in service corridors.

Level 2

In garages, warehouse storage areas and other similar areas where thefinal surface appearance is not of concern, a Level 2 finish is therecommendation. Level 2 may be specified where moisture resistant gypsumboard is used as a tile substrate. Level 2 now reads, “All joints andinterior angles shall have tape embedded in joint compound and wipedwith a joint knife leaving a thin coating of joint compound over alljoints and interior angles”. This change is to further differentiateLevel 2 from Level 1. Joint compound is applied over all fastener headsand beads. The surface is left free of excess joint compound. Ridges andtool marks are acceptable for a Level 2 finish. Additionally, Level 2now includes the following sentence: “Joint compound applied over thebody of the tape at the time of tape embedment shall be considered aseparate coat of joint compound and shall satisfy the conditions of thislevel.” This sentence also appears in the ASTM C840 Appendix. In thepast there has been some confusion as to whether tape pressed into jointcompound and covered with joint compound in a single operation fulfilledthe requirements of Level 1 or Level 2. This statement is intended toclarify the requirements of Level 2.

Level 3

In areas to be decorated with a medium or heavy texture or whereheavy-grade wall coverings will become the final decoration, a Level 3finish is recommended. Level 3 now states, “All joints and interiorangles have tape embedded in joint compound and one additional coat ofjoint compound applied over all joints and interior angles. Fastenerheads and accessories shall be covered with two separate coats of jointcompound. All joint compound shall be smooth and free from tool marksand ridges.” Before final decoration it is recommended that the preparedsurface be coated with a drywall primer prior to the application offinal finishes. Level 3 is not recommended where smooth painted surfacesor light- to medium-weight wall coverings become the final decoration.

Level 4

If the final decoration is to be a flat paint, light texture orlightweight wall covering, a Level 4 finish is recommended. Level 4 hasbeen modified to read, “All joints and interior angles have tapeembedded in joint compound and two separate coats of joint compoundapplied over all flat joints and one separate coat of joint compoundapplied over interior angles. Fastener heads and accessories shall becovered with three separate coats of joint compound. All joint compoundshall be smooth and free from tool marks and ridges.” Before finaldecoration it is recommended that the prepared surface be coated with adrywall primer prior to the application of final finishes. Gloss,semi-gloss and enamel paints are not recommended over a Level 4 finish.

A level 5 finish is defined as follows: “Level 5 finish is recommendedfor areas where severe lighting conditions exist and areas that are toreceive gloss, semi-gloss, enamel or non-textured flat paints. Level 5requires all the operations in Level 4. Additionally, a thin skim coatof joint compound, or material manufactured especially for this purpose,is applied to the entire surface. (This definition is referenced toTerminology, Section II, Page 2 of GA-214 to make the description of“skim coat” clear to all.) The surface is smooth and free from toolmarks and ridges. Before final decoration it's recommended that theprepared surface be coated with a primer prior to the application offinal finishes. The Level 5 finish is required to achieve the highestdegree of quality by providing a uniform surface and minimizing thepossibility of joint photographing and/or fasteners “burning through”the final decoration.”

According to one aspect of the present invention, a level 5 finish canbe achieved on a gypsum board during manufacture of the board and priorto installation on a wall. According to another aspect of the presentinvention, a level 4 finish can be achieved on a gypsum board duringmanufacture of the board and prior to installation on a wall.

According to yet another aspect of the invention, a wall can beassembled by producing a gypsum board and coating the gypsum boardbefore the drying step with a joint compound or a diluted jointcompound. And, when the gypsum board is fastened to a supportingstructure to form the wall, the seams between adjacent boards can betaped and finished using a joint compound having a substantially similaror identical composition to the coating. Any of the coatings disclosedherein may be used on embodiments of the present invention.

According to a preferred embodiment, at least one jointing material andat least one skim coat on the board exhibit substantially the samesurface water absorbability. Preferably, the at least one jointingmaterial and the at least one skim coat exhibit also substantially thesame coloration and/or reflectance factor.

According to a preferred embodiment, the at least one jointing material,e.g. the joint-pointing coat, and the at least one skim coat havesubstantially the same solids formulation, the skim coat when applied tothe board having a solids content lower than the jointing material whenapplied. Especially, when the skim coat is applied, the viscosity willbe adjusted to the viscosity needed for the coating apparatus that willbe used.

Compared to the jointing material such as the joint-pointing coat, theskim coat, when applied (for example, when the skim coat is applied tothe prefabricated elements, or when the skim coated prefabricatedelements are assembled in the methods of the present invention), willcomprise more water than initially present in the jointing material suchas the joint-pointing coat. It will thus generally comprise additionalwater, e.g., about 10 to 60%, preferably about 15 to 40%, morepreferably about 25%, more water than initially present in the jointingmaterial such as the joint-pointing coat. By “water initially present inthe jointing material”, it means the amount of water present in thejointing material when the jointing material is applied to the skimcoated prefabricated elements, before the jointing material is in a drystate.

In one embodiment, the final thickness of the skim coat (determined whendry) is generally between about 0.001 and about 3 mm, preferably betweenabout 0.01 and about 2 mm, more preferably between about 0.01 and about1 mm, and most preferably between about 0.2 and about 1 mm.

According to another preferred embodiment, the jointing material such asthe sealing coat, joint coat and/or joint-pointing coat and the skimcoat have the following composition, by weight:

40 to 70% of a mineral filler, such as calcium carbonate;1 to 10% of hydrophobic surface perlite;0.1 to 10% of a binder such as polyvinyl acetates and acrylic acidesters in aqueous solution;0.1 to 10% of a handling agent such as a cellulosic ether;0.1 to 5% of a slipping agent such as clay;0.1 to 20% of another silicate derivative as an additional slippingagent, such as talc and mica;water with optional conventional additives up to 100%.The compositions of the jointing material such as the joint-pointingcoat and the skim coat are not limited to the components disclosed abovein the above amounts for the preferred embodiment. These components maybe present in other amounts in the compositions of the jointing materialsuch as the joint-pointing coat and the skim coat. By using theembodiment involving the skim coat, it is possible to obtain the samesurface water absorption, reflectance factor and/or color deviation aswith the first embodiment depicted above, i.e. the embodiment involvingadapting the composition of the jointing material, e.g. the sealingcoat, joint coat and/or joint-pointing coat, to the composition of thesurface of the prefabricated elements, e.g. the upper layer or weband/or the pigment layer of the plaster boards, to form a substantiallyhomogeneous outer surface in the dry state.

According to one preferred embodiment, a gypsum board is produced byforming a gypsum slurry and depositing the slurry onto a facing sheetlaid out on an endless conveyor belt. A backing sheet is then laid ontop of the gypsum slurry. The facing sheet, the gypsum slurry, and thebacking sheet are then formed into a wet gypsum board by passing thegypsum and optional facing and backing sheet through a forming roller orrollers to produce the desired thickness. The wet gypsum board iscarried by a conveyor belt until a set time expires, after which thegypsum board is cut into desired lengths with a blade cutter, and thenpassed through a mechanical coater. A subsequent application of heat,such as in a dryer or an oven, removes excess water. At any stage duringthe process, either the facing sheet or the backing sheet or both may beremoved.

In an alternative embodiment of the present invention, the gypsum boardproduct can be formed in a manufacturing line generally known for makingceiling tiles. In such a process, the desired components are blendedwith a high shear mixer in a batch. The batch is then extruded throughan opening of an extruder onto an endless conveyor belt. After apredetermined set time, the mixture is then cut into desired lengths.The cut lengths are then passed through a mechanical coater followed bya dryer to evaporate the excess water.

According to an embodiment of the present invention, lightweight gypsumboards can be made. The density of such boards can range from as low as1,000 lbs. per thousand square feet to about 1,700 lbs. per thousandsquare feet, or greater. After cutting the gypsum and while the gypsumis still not dry, the wet gypsum board is transported to the coating anddrying line. A coating of thickness of up to 30 mils is applied which issubsequently dried in a drying process, e.g., dried in an oven.

In one preferred embodiment, at least one parameter differs by no morethan 10%, preferably no more than 5%, at different parts of the overallsurface, so that at least 90%, preferably at least 95%, homogeneity isachieved. In other words, as an example, when said at least one jointingmaterial and the prefabricated elements are dry, said at least one ofthe parameters of the visible surface of said at least one jointingmaterial can differ by not more than 10%, preferably not more than 5%,from the corresponding parameter of the visible surface of theprefabricated elements.

According to another embodiment of the present invention, a gypsum boardproduct of a desired thickness can be made as conventionally known. Forexample, a ⅝ inch, ½ inch or ¼ inch gypsum board can be made. Aftercutting the gypsum and while the board is still not dry, the wet gypsumboard is transported to the coating and drying line. A coating ofthickness of up to 30 mils is applied which is subsequently dried in adrying process, e.g., dried in an oven.

Although the present invention has been described in connection withpreferred embodiments thereof, it will be appreciated by those skilledin the art that additions, deletions, modifications, and substitutionsnot specifically described may be made without department from thespirit and scope of the invention as defined in the appended claims.

1.-27. (canceled)
 28. A method of producing a coated gypsum board, themethod comprising: depositing a gypsum slurry to form a wet gypsum boardhaving a gypsum core, applying a coating to the wet gypsum board; anddrying the wet gypsum board, wherein the step of applying the coatingoccurs prior to the step of drying and the coating penetrates through atleast a portion of the facing sheet and into at least a portion of thegypsum core.
 29. The method of claim 28, wherein the coating is applieddirectly to the gypsum core.
 30. The method of claim 28, wherein thegypsum slurry is deposited onto a facing sheet and wherein the coatingis applied to the facing sheet.
 31. The method of claim 30, furthercomprising placing a backing sheet opposite the facing sheet with thegypsum core therebetween.
 32. The method of claim 28, wherein thecoating is applied during a setting period of the gypsum board.
 33. Themethod of claim 30, wherein the coating is a diluted joint compound. 34.A method of producing a coated gypsum board, the method comprising:depositing a gypsum slurry to form a wet gypsum board having a gypsumcore, applying a coating to the wet gypsum board; and drying the wetgypsum board, wherein the step of applying the coating occurs prior tothe step of drying and the coating is a diluted joint compound.
 35. Themethod of claim 34, wherein the coating is applied directly to thegypsum core.
 36. The method of claim 34, wherein the gypsum slurry isdeposited onto a facing sheet and wherein the coating is applied to thefacing sheet.
 37. The method of claim 36, further comprising placing abacking sheet opposite the facing sheet with the gypsum coretherebetween.
 38. The method of claim 34, wherein the coating is appliedduring a setting period of the gypsum board.
 39. The method of claim 36,wherein the coating penetrates through at least a portion of the facingsheet and into at least a portion of the gypsum core.
 40. A method ofproducing a coated gypsum board, the method comprising: depositing agypsum slurry to form a wet gypsum board having a gypsum core, applyinga coating to the wet gypsum board; and drying the wet gypsum board,wherein the step of applying the coating occurs prior to the step ofdrying and coated board has a level 5 finish prior to assembly in itsintended use.
 41. The method of claim 40, wherein the coating is applieddirectly to the gypsum core.
 42. The method of claim 40, wherein thegypsum slurry is deposited onto a facing sheet and wherein the coatingis applied to the facing sheet.
 43. The method of claim 42, furthercomprising a step of placing a backing sheet opposite the facing sheetwith the gypsum core therebetween.
 44. The method of claim 40, whereinthe coating is applied during a setting period of the gypsum board. 45.The method of claim 42, wherein the coating penetrates through at leasta portion of the facing sheet and into at least a portion of the gypsumcore.
 46. The method of claim 42, wherein the coating is a diluted jointcompound.
 47. A method of producing a coated gypsum board, the methodcomprising: depositing a gypsum slurry to form a wet gypsum board havinga gypsum core, applying a coating to the wet gypsum board; and dryingthe wet gypsum board, wherein the step of applying the coating occursprior to the step of drying and the thickness of the coating is up to 30mils.
 48. The method of claim 47, wherein the coating is applieddirectly to the gypsum core.
 49. The method of claim 47, wherein thegypsum slurry is deposited onto a facing sheet and wherein the coatingis applied to the facing sheet.
 50. The method of claim 49, furthercomprising placing a backing sheet opposite the facing sheet with thegypsum core therebetween.
 51. The method of claim 47, wherein thecoating is applied during a setting period of the gypsum board.
 52. Themethod of claim 49, wherein the coating penetrates through at least aportion of the facing sheet and into at least a portion of the gypsumcore.
 53. The method of claim 49, wherein the coating is a diluted jointcompound.